A novel approach for the placement of zygomatic implants: the zygomatic bone zone index

The minimum amount of bone required for a dental implant is a relevant topic, not just for implant dentists, but also for general dentists, oral surgeons, prosthodontists, periodontists and any other health professional who provides treatment planning advice to patients about implants.

The minimum height of vertical bone required for the placement of dental implants in the posterior maxilla was originally defined by the Sinus Consensus Conference in 1996.¹ The consensus stated that a residual bone height inferior to the sinus floor of 3 mm or less is best treated with a lateral sinus graft approach, with a delay in implant placement. In 2008, 2011 and 2016, the International Team for Implantology (ITI) clarified this position, stating that a residual height of bone inferior to the sinus of 4–6 mm justifies the provision of a direct lateral sinus graft, with simultaneous implant placement.² In 2013, the ITI considered that a minimum of 1 mm of bone circumferentially around the planned implant was required before considering bone grafts.³ This would equate to a ridge width of 5 mm for an implant that is 3 mm wide.

Therefore, a posterior maxilla with less than 3 mm residual bone height below the sinus and/or 4 mm or less bucco-palatal width of bone prevents the placement of conventional implants and could be considered ‘extremely’ atrophic. An edentulous ridge such as this would require bone grafting to allow implant placement, or alternatively the use of an implant anchored in the bone outside the maxilla, otherwise known as ‘extra-maxillary implants’. Zygomatic implants are a type of extra-maxillary implant that is used to support teeth when there is an extremely atrophic ridge.

Zygomatic implants

Brånemark first wrote about zygomatic implants in 1998 after using them for 10 years.⁴ This implant is a longer titanium rod that is apically installed into the zygomatic bone, but emanates from the maxillary crest (Figure 1). Primary stability of the zygomatic implant is often excellent and can allow for immediate placement of a fixed prosthesis.^5,6

Since the late 1990s zygomatic implants have been used in cases with severe bony atrophy or tissue loss. Soft and hard tissue atrophy from tooth loss or previous implant failure can lead to an unsuccessful complete denture owing to a lack of retention and support being provided from the surrounding tissues. Zygomatic implants are often installed to support and retain a fixed or removable prosthesis. Although this is a common reason for zygomatic implants to be installed, it is not the only one. Genetic syndromes such as ectodermal dysplasia, where teeth are naturally missing, leads to a poorly developed maxilla with similar issues for complete denture construction and often, zygomatic implants are used in these cases. Other reasons for maxillary bone to be missing or poorly developed can be trauma, abnormal development, such as cleft palate, or from sinister developments, such malignant tumours that require bony resections of the maxilla and other facial bones.⁷ Once zygomatic implants are placed in these types of cases, they will commonly support various types of conventional implant prosthetics, such as fixed bridges or removable overdentures, but will occasionally aid facial and oral obturators and prosthetics.⁸

Zygomatic implant surgery is not without its complications. The surgery is invasive and can cause trauma to surrounding vital structures. Oral antral fistulas and sinusitis are not uncommon, and can develop owing to the zygomatic implant's trajectory through the maxillary sinus cavity. Some of the more severe complications described within the literature relate to significant trauma of the orbit⁹ or damage to the neurovascular vessels, leading to bleeding and nerve damage.¹⁰ Complications are often due to a lack of good visual and surgical access and misjudgement of the drilling trajectory.¹¹

The current literature and the problem

Although zygomatic implants have been used clinically since the 1990s, and high survival rates of over 96% have been reported,¹² there are limited data for understanding the anatomy of the zygomatic bone and the ideal apical placement of the implant. Currently, there is little guidance on where to place the apex of the implant in the zygomatic bone in relation to the area of prosthetic support required in the maxilla. Aparicio et al's zygoma anatomy-guided approach (ZAGA) relates the apical position to ‘complement the implant path’ while maintaining 3 mm of bone over the apical threads.⁵ They also found that the choice of apical positioning is dependent on the curvature of the zygomatic process and the number of zygomatic implants to be placed on the day of surgery.

Background

3D radiography is required to consider and assess whether it is possible to install implants in areas such as the zygomatic bone region. It is essential to visualize the anatomy of the zygoma, and to assess whether there is adequate space for one or two implants to be installed and whether any vital structures lie in the path of the implant trajectory. The 3D radiological view of the zygoma would then determines whether zygomatic implants are an option, or if grafting may be more appropriate owing to potential risks, or lack of zygomatic bone.

Current literature shows differences in the original technique taught by Brånemark et al,¹³ and there have been reports of cases with one, two and even three implants installed into each zygoma.¹⁴

Regardless of what surgical technique may be used, proper planning of apical positioning is essential to avoid any significant complication, such as inadvertent trauma to the orbit, or entrance into the infratemporal fossa, caused by a lack of space and improper drilling technique.¹⁵

Chrcanovic and Abreu raised an important point in 2013.¹¹ If one zygomatic implant is to be installed in the zygomatic bone, it is be prudent to consider future planning and ensure that there is sufficient space in the remaining zygomatic bone to accommodate and install further zygomatic implants. Strategic planning of the spatial requirements for current and future zygomatic implants is also required in case of potential failure of the newly installed zygomatic implant, or even failure of the conventional implants that may have been used in conjunction with the zygomatic implants (Figure 2).

The planning of the apical zygomatic points should follow the work by Rigolizzo et al¹⁶ (Figure 3), Takamaru et al¹⁷ and Hung et al,¹⁸ who all showed multiple appropriate areas to engage the implants into the zygomatic bone. Their research reveals that in almost all cases, there is sufficient bone to place implants into the central body of the zygoma. Based on their findings,^16,17,18 the body of the zygomatic bone to be used for implant placement can be simplified into four zygomatic bone zones, A, B, C and D, for ease of surgical planning (Figure 4).

Quilez et al¹⁹ confirmed that for every patient who presented to their team, there was the possibility to install at least two zygomatic implants into these four specific zygomatic bone zones, which provided each patient with a total of four zygomatic implants (quad zygo).

The Zygomatic Bone Zone index

This article introduces a treatment tool to simplify the planning of the apical positioning of the zygomatic implant in relation to the required maxillary support: the Zygomatic Bone Zone index.

With this simple index, support to the posterior maxilla with the apex of the implant engaging the inferior zygomatic bone in sections C and D can be envisaged and planned. Additionally, if there is a need for a zygomatic implant to support the anterior maxilla, the apex can be planned to engage the superior aspect of the zygomatic bone (zones A and B; Figures 5–7). Keeping to this index will help to avoid situations where the superior part of the zygomatic bone has been used for a posterior zygomatic implant (Figure 2), which would necessitate installation of any further zygomatic implant even closer to the orbit, or at worst, would prevent a further zygomatic implant. Conversely, an anterior zygomatic implant installed too low on the zygomatic bone also prevents further installation of zygomatic implants in the posterior maxilla because the existing anterior zygomatic implant will be within the trajectory of any potentially new zygomatic implant to be installed posteriorly.

The apical position should also ideally be kept within these four zones, as posterior to this zygomatic area, the bone is thinner and there is an increased chance of damage to the infratemporal region. To avoid trauma to the orbital rim, the apex should be planned to be 3 mm away from the orbital rim.

The Zygomatic Bone Zone index can be used by both experienced and inexperienced zygomatic implant surgeons to future-plan implant positions according to the available bone, prosthetic position and most importantly, in case of future implant failure, to ensure that there is adequate zygomatic bone for further implants. It can also help determine which patients may be more challenging to provide a ‘quad zygomatic’ implant treatment in relation to their nearby vital structures.²⁰

Discussion

Although 3D imagery is a standard requirement for the assessment of zygomatic implants, the specific apical positioning of these implants and future planning for further implants are not. Occasionally zygomatic implants are placed directly through the full body of the zygomatic bone preventing further placement if another zygomatic implant is required later. Standardization and following a structured positioning plan will help to avoid this conundrum, and also prevent entrance into the orbit and infratemporal fossa and avoid other structures.^21,22

Based on the existing available literature, the present author proposes a new simple assessment method for the zygomatic bone to anchor implants for the extremely atrophic maxilla. The aim is to help surgeons form a safe and structured approach to treatment planning. This article sets forward that the ‘extremely’ resorbed maxilla may be defined as:

• 4 mm or less bucco-palatal bone width; and/or
• 3 mm or less bone height below the sinus

In scenarios with this degree of atrophy, zygomatic implants may be an appropriate treatment option rather than bone grafting.

The zygomatic implant supporting the posterior maxilla could be installed in the inferior aspect of the zygoma in zones C and D, while an anterior zygomatic implant that is planned to support anterior teeth could be installed in the superior aspect of the zygomatic bone in zones A and B. This planning avoids spatial issues and encroachment into the vital structures, but also prevents a single zygomatic implant with a trajectory through the full zygomatic body that would prevent future zygomatic implants should they be required.

The new simplified structured planning, which separates the zygoma into four distinct parts, will assist surgeons in planning the appropriate apical position for an implant, for the appropriate atrophic edentulous maxillary zone that requires support for a prosthesis. It will also ensure that patients who have minimal zygomatic bone regions will be highlighted during the assessment.²³

This tool, the Zygomatic Bone Zone index, will help to standardize and structure cases for the apical positioning of this complex implant therapy. The tool is designed to improve the safe provision of at least one or two implants in each zygomatic bone by accounting for the current and future spatial bony requirements. It will also provide a basis for teaching the zygomatic implant modality. To the author's knowledge, this is the first classification to separate the zygomatic bone into zones according to the support required from the anterior or posterior maxilla.